Visual Cell X/Y Classifications: Characteristics and Correlations
Investigators have used a battery of tests for a variety of characteristics to classify retinal ganglion and lateral geniculate nucleus (LGN) cells as X or Y. The characteristics that have been tested include: linearity of spatial summation, response dynamics (temporal frequency tuning), action potential conduction velocity (axon diameter), receptive field center size (spatial frequency tuning), dendritic field size, cell body size, and receptive field eccentricity in the visual field. The methods and results of these measurements are reviewed here, with special attention given to evidence on two issues. Are the results clearly bimodal separating distinct groups, or is the grouping a result of an arbitrary division of a continuum? What are the correlations among the classifications according to different characteristics—that is, are there really two distinct groups (X/Y) or are many cells found with mixed characteristics? Evidence is presented suggesting that in general the distributions are not strictly bimodal. Furthermore, the results of new experiments where many of the characteristics above were tested for each LGN unit from which recordings were made, indicate that the correlations among the different characteristics are weak, though positive. Thus, although a tendency for correlation does appear, a cell that is identified as X by a single characteristic test may be classified as Y when tested for another characteristic.
KeywordsReceptive Field Conduction Velocity Retinal Ganglion Cell Lateral Geniculate Nucleus Conduction Time
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